Signal and data processing systems often employ analog-to-digital converters (ADCs) for converting an analog signal (e.g., an audio signal received via a microphone, an analog signal received with radio-frequency communication, etc.) into a corresponding digital signal which may be further processed (e.g., by a digital signal processor). In many applications, an ADC may be implemented using a delta-sigma converter. Delta sigma converters are subject to intersymbol interference (ISI), mismatched levels (in the case of multilevel quantization), mismatched components (typically in a feedback digital-to-analog feedback path of an ADC), and various other hardware-based imperfections that limit accuracy and may vary from converter to converter.
Numerous approaches have been employed to compensate or correct for the imperfections of delta-sigma converters. For example, U.S. Pat. No. 6,449,569 to John Laurence Melanson (the “'569 reference”) discloses signal processing techniques for correcting distortion in a delta-sigma modulator. The technique disclosed in the '569 reference relies on the natural behavior of an ADC to generate operating modes that allow correction to take place. FIG. 1 depicts an example continuous-time ADC 100, as is known in the art. An analog input signal may be input to a loop filter 102 (which may be implemented using a delta-sigma modulator), the output of which is quantized by a quantizer 104 into a digital output signal having N bits. If thermometer coding is used for the digital output signal, the digital output signal will have N levels, wherein the level of the digital output signal is indicated by the number of bits of the N-bit digital output signal which are asserted. In the feedback path of ADC 100, a dynamic element matching block 106 may rearrange bits of the digital output signal to generate an equivalent N-bit signal communicated to feedback digital-to-analog converter (DAC) 108. In a thermometer coding implementation, DAC 108 may convert the digital signal into an analog feedback signal that is fed back to loop filter 102 by buffering each bit with a buffer 110 and driving the buffered signal through a corresponding resistor 112, such that the analog feedback signal has a magnitude corresponding to the number of asserted bits of the feedback signal. Ideally, to generate an accurate analog feedback signal, resistors 112 will have identical impedances. However, due to process or temperature variances, their impedances may differ. Without correction for the mismatch in resistors 112, ADC 100 may have an unacceptable amount of non-linearity. While DEM block 106 may effectively convert the resistor mismatched-induced distortion into white noise, many existing DEM techniques may aggravate inter-symbol interference (ISI) error.